The present invention concerns an apparatus for collecting ground radar data with polarization information.
In the following, polarization data is taken to mean including information on the electrical field direction of the radar wave.
Ground penetrating radar (GPR), also called ultra wideband radar, is a kind of time-dependent radar system characterized by its high bandwidth. Commercial GPR system have a typical bandwidth in excess of 1.
To study the characteristics of a land area or more generally a sample medium a radar system is used in which one antenna transmits electromagnetic signals and another antenna registers the signals that are reflected from the sample medium. The information that is registered can be analysed to establish the characteristics of the sample medium and to establish the position of objects enclosed in the medium. The transmitted signals will change depending on the construction of the radar system. Several transmitters and receivers can be used in a multi-channel system for special applications.
The transmitted electromagnetic waves have a number of properties that change as a result of their passage through the sample medium, reflection against objects and reflection against boundaries within the sample medium. These properties comprise amplitude, frequency, phase and polarization and are used to give a better picture of the conditions within the sample medium.
A linear or generally more linear-like reflector polarizes all the incoming electromagnetic fields so that they will be roughly parallel to the reflector after reflection. As a result of this, a radar system using linear polarized antennas will register maximum amplitude when both the transmitter and receiver antennas are oriented parallel to the linear reflector. Minimum amplitude is registered when both antennas are oriented perpendicular to the reflector. Intermediate orientations, between parallel and perpendicular, will give intermediate amplitudes. With different orientations of transmitter and receiver antennas, a number of additional cases can be achieved. All of these situations can be analysed qualitatively by ascertaining that maximum coupling to a linear reflector is obtained when the antenna is parallel to the reflector, irrespective of whether it is a transmitter or a receiver.
From U.S. Pat. No. 6,388,629 is known an apparatus that uses one or more rotating antennas with both transmitter and receiver elements. The antennas are contained in a housing and rotate relative to the ground and consequently also relative to the apparatus itself. The apparatus is located on a mobile unit used to move the apparatus along the ground. The apparatus also comprises a pulse generator, which is electrically connected to each transmitting element for the transmission of a radar pulse from one antenna and an A/D converter included in a sampler unit that is electrically connected to each receiver element for receiving radar pulses from the antenna. The device also comprises a data unit for storing collected readings, a power source and a control unit. For transferring signals between the said transmitter and receiver units and the A/D converter in the sampler unit, the apparatus comprises a slip-ring arrangement that acts between the antennas and the other units contained in the apparatus.
Thanks to the aforesaid type of ground radar apparatus with rotating antennas, it has been proven possible to achieve both improved signal quality and reduced sensitivity to interference.
However, the ambition is always to further improve both of these parameters, i.e. to improve signal quality and reduce sensitivity to interference.